AD&D Rule Spotlight: Casting Spells Directly from Books
This rule is simple. In a pinch, you can cast a spell straight from the page. It acts like a scroll. You pay for it.
The spell you cast is gone from the book. There is a small chance the spells on either side are gone too. There is also a tiny chance the whole book dies. Permanency does not save it. A backup copy at home looks smart now.
I like the tension it brings. The Magic-User can solve the problem right now, but they put their book on the line. That is old school play. Choices have weight.
Keep the rulings tight. You must be able to cast the spell. You must have the right kind of book. Blank pages do not protect neighboring spells. Travel books are fine, just keep them honest and numbered.
At my table, a player tried to beat the system with empty pages. I ruled no. The risk stays with the next and previous spells. He used the book anyway.
Use this rule when you want heat. It turns a page into a resource and a decision into a moment.
The more I hear about these ways of spell casting, the more I consider why not just have scrolls, so a spell book is nothing more than an elaborate "ring binder" of accumulated scrolls, each of which can be used to memorise (and remain) or can be cast from directly (and vanish), level limits apply so a spell caster can have a scroll with a spell they cannot memorise, perhaps waiting to when they do reach the appropriate level, and hopefully don't need to cast it directly in the meantime.
The availability of scrolls is normally limited, but this can be rectified by implying any enemy spell casters that are defeated will have scrolls of the spells they know somewhere in their treasure.